T9B05: What generally happens as the frequency of a signal passing through coaxial cable is increased?
A. The apparent SWR increases
B. The reflected power increases
C. The characteristic impedance increases
D. The loss increases
Coaxial cable is the most commonly used type of amateur radio transmission line, or feedline, because it is easy to use and requires few special installation considerations. Coaxial cable can be effectively used from HF through VHF and UHF frequencies common in amateur radio, but every ham should be familiar with the characteristics of coax and understand its limitations, particularly as frequency increases into the UHF range. That’s what this question is getting at.
Coaxial cable can impose some significant loss in signal strength compared to other types of feedlines, such as ladder line or twin-lead. There are two primary causes of signal loss with coax: 1) Resistive loss, and 2) Dielectric loss.
Resistive loss arises from the resistance of the center conductor of the cable and results in dissipated heat. Radio frequency currents tend to flow along and near the surface of conductors. This is known as the skin effect. The greater the surface area of a conductor, the lower the resistance, and vice versa – small surface area means higher resistance.
As signal frequency increases the skin effect becomes more pronounced. That is, the “depth” within the conductor through which currents flow becomes reduced as frequency increases. As these higher frequency signals are squeezed closer and closer to the surface of the conductor, the effective cross-section area of the conductor through which they may flow is decreased. When the current flow area is decreased the electrical resistance increases. (Think of the water analogy, in which the pipe is narrowing through which the current may flow.) Resistive loss has a significant effect even down in the HF frequency range.
Dielectric loss refers to signal loss resulting from some complex interactions of the alternating electric field with the insulating material that separates the center conductor from the surrounding shield in the coaxial cable. Generally, the more dense the insulator the greater the dielectric loss that results.
For frequencies in the HF range, dielectric loss is usually not significant for most coaxial cable constructions. However, as frequencies increase into the VHF range, and even more significantly into the UHF range, dielectric loss becomes quite significant with most coax insulating materials.
So, as signal frequency increases the loss increases due to both resistive and dielectric loss characteristics. But resistive loss dominates in the HF range, and dielectric loss begins to become significant in the VHF range. In the UHF range these two characteristics are adding together to produce large loss values in many types of coax.
Remember, the denser the dielectric insulating material the greater the dielectric loss. And the lower the conductor surface area (smaller diameter conductor) the greater the resistive loss. So, a very low density insulating material, such as air, combined with a large surface area center conductor, such as a large diameter multi-strand copper conductor, will produce the lowest loss figures for coax. The air insulated hard line provides just such a combination and the lowest loss characteristics of any type of coaxial cable.
Signal loss is measured in decibels of power. Below is a table of some typical coax types and their loss figures for given frequencies. Each value is the loss in decibels per 100 feet of coax length. Notice how in each cable type the loss per 100 feet increases with signal frequency, and notice that the magnitude of increase gets larger in the 400 MHz (UHF) range.
The answer to Technician question T9B05, “What generally happens as the frequency of a signal passing through coaxial cable is increased?” is D: The loss increases.
Related Questions: T9B01, T9B02, T9B03, T9B08, T9B10, T9B11.